1. Field of the Invention
The present invention relates in general to a structure of a gas sensor, and more specifically relates to a gas sensor for detecting characteristics of a gas existing in a predetermined flow path.
2. Description of Related Art
Conventionally, a gas sensor measures characteristics (for example, a concentration, a temperature, or a humidity) of a gas existing in a flow path using a detection element. In such a gas sensor, a signal from the detection element is electrically processed and outputted as an electric signal corresponding to the characteristics of the gas. In one example, an ultrasonic type gas sensor, which is provided in transportation equipment with an internal combustion engine (such as an automobile) detects a concentration of gasoline, gas oil, or the like utilizing a change in a propagation speed of an ultrasonic wave. Such a gas sensor is provided, for example, in the middle of a purge line that is connected from a canister mounted on the automobile to an inlet pipe of the internal combustion engine. An evaporated fuel gas containing gasoline or the like passes through a flow path of a predetermined volume formed in the sensor. When a concentration of the gasoline vapor changes, a speed of an ultrasonic wave passing through a medium changes, and this speed change is detected by a receiver of the ultrasonic wave to process a signal and output as a signal corresponding to a gasoline concentration. Usually, the time it takes for the ultrasonic wave outputted from a transmitter to propagate a predetermined distance to reach the receiver is detected to find the gasoline concentration.
Among conventional sensors, there are a few that can directly convert a change in characteristic of a gas into a large electric signal, and an electric signal outputted from the detection element is often feeble. Consequently, an output may change even if a slight force acts on the detection element, and in order to avoid this, a measure is taken such as molding an element main body to be fixed with resin in a conventional detection element. For example, in the above-described ultrasonic type gas sensor, it is a general practice to, after housing an element for transmitting or receiving an ultrasonic wave in a dedicated housing case, filling resin, for example, urethane resin in the housing case to embed and fix the detection element (for example, see JP-A-2000-206099).
However, in such a gas sensor, there is a problem in that a stress is applied to a detection element (such as a vibration element) for detecting an ultrasonic wave due to thermal expansion or the like of molded resin to decrease a detection accuracy, or in some cases, the vibration element is displaced due to the thermal expansion of resin to affect the detection accuracy. For example, in the above-described gas sensor using an ultrasonic wave, in order to isolate an element for transmitting or receiving an ultrasonic wave from a gas which is an object of detection, in some cases, a thin film is provided in an opening portion of a housing case and, after attaching the element on the thin film, the housing case is filled with resin. When such a structure is adopted, a filler inside the housing case of the vibration element thermally expands at the time of high temperature and a stress is generated in the inside thereof. As a result, a phenomenon is observed in which a part of the vibration element projects from the opening portion of the housing case while the thin film is deformed in such a manner as being pushed out by the expanded filler. When the part of the vibration element projects, a propagation distance of an ultrasonic wave also changes, which affects a detection accuracy.
In particular, in the case in which a vibration element (i.e. the detection element) is used for both transmission and reception of an ultrasonic wave, a problem has been pointed out in that an ultrasonic wave reflects around the vibration element (for example, a side or the like of a case of the detection element), whereby noises are generated. An ultrasonic wave which has propagated in a direction other than in a direction along a flow path for detection from the vibration element may reflect on an interface, where a difference of densities of media is large, and then return to the vibration element. This is observed as a noise. If noises are generated in a large quantity and for a long time, it is possible that the noise affects a detection accuracy of an ultrasonic wave.